This invention relates to intercalated graphite flake having increased exfoliation volume at temperatures as low as 600xc2x0 C. and even lower.
Graphite is a crystalline form of carbon comprising atoms bonded in flat layered planes with weaker bonds between the planes. By treating particles of graphite, such as natural graphite flake, with an intercalant of, e.g., a solution of sulfuric and nitric acid, the crystal structure of the graphite reacts to form a compound of graphite and the intercalant. The treated particles of graphite are hereafter referred to as intercalated graphite flake. Upon exposure to elevated temperatures the particles of intercalated graphite expand in dimension in an accordion-like fashion in the c-direction, i.e. in the direction perpendicular to the crystalline planes of the graphite.
Intercalated graphite flake has many useful applications. A common application is to exfoliate the intercalated graphite particles into vermicular-like structures which are then compressed into sheets of flexible graphite for use in the manufacture of gaskets or as packing material. Intercalated graphite flake is also used in a variety of products which take advantage of the high expansion characteristic of intercalated graphite flake when exposed to high temperature. One such example is for use in combination with polymer foams to form seat cushions and furniture upholstery in aircraft. Upon exposure to fire, the high temperature will cause the particles of intercalated graphite to exfoliate which minimizes or prevents the formation of toxic gases from the polymer foam and may, of itself, smother a fire.
Since it is important to suppress, i.e. retard a fire before it has begun to spread, it would be a substantial advantage for an intercalated graphite flake product to exhibit a very high degree of exfoliation upon exposure to temperatures as low as 600xc2x0 C. and even lower.
It has been discovered in accordance with the present invention that the addition of an organic expansion aid to the intercalation solution and the treatment of intercalated graphite flake with an organic reducing agent, following intercalation of the graphite flake with an oxidizing intercalant solution, and while the graphite flake is covered with a coating of oxidizing intercalant solution, results in a material which exhibits enhanced exfoliation volumes at exfoliation temperatures as low as 600xc2x0 C. and even lower.
The method of the present invention for forming particles of intercalated graphite flake having enhanced exfoliation volume at temperatures as low as 600xc2x0 C. and even lower by:
(a) adding an organic expansion aid to an oxidizing intercalant solution;
(b) treating particles of graphite with the oxidizing intercalant solution containing the expansion aid to provide intercalated graphite flake with a surface film of oxidizing intercalant solution;
(c) contacting the surface film of the intercalated graphite flake with an organic reducing agent in the form of an organic compound selected from sugars, alcohols, aldehydes and esters which is reactive with the film of oxidizing intercalant solution at temperatures in the range of 25xc2x0 C. to 125xc2x0 C.; and
(d) subjecting the thus treated intercalated graphite flake to a temperature in the range of 25xc2x0 C. to 125xc2x0 C. to promote a reaction of the organic reducing agent with the surface film of oxidizing solution.
Intercalated graphite flake is conventionally formed. by treating particles of natural graphite with agents that intercalate into the crystal structure of the graphite to form a compound of graphite and the intercalant capable of expansion in the c-direction, i.e. the direction perpendicular to the crystalline planes of the graphite, when heated to a high temperature of above 700xc2x0 C. and preferably above 1000xc2x0 C. The intercalated graphite flake is washed and dried prior to exfoliation. Exfoliated graphite particles are vermiform in appearance and are commonly referred to as xe2x80x9cwormsxe2x80x9d.
A common conventional method for forming intercalated graphite flake (and for manufacturing sheets of flexible graphite from exfoliated graphite) is described in U.S. Pat. No. 3,404,061 the disclosure of which is incorporated herein by reference. As disclosed in the above mentioned patent natural graphite flake is intercalated by dispersing flakes in a solution containing an oxidizing agent, such as a mixture of nitric and sulfuric acid. After the flakes are intercalated excess solution is drained from the flakes. The quantity of intercalation solution retained on the flakes after draining is typically greater than 100 parts of solution by weight per 100 parts by weight of graphite flakes (pph) and more typically about 100 to 150 pph.
The intercalant of the present invention contains oxidizing intercalating agents known in the art. Examples include those containing oxidizing agents and oxidizing mixtures, such as solutions containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like, or mixtures, such as for example, concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or mixtures of a strong organic acid, e.g. trifluoroacetic acid, and a strong oxidizing agent soluble in the organic acid.
In the preferred embodiment of the invention, the intercalant is a solution of sulfuric acid, or sulfuric acid and phosphoric acid, and an oxidizing agent, i.e. nitric acid, perchloric acid, chromic acid, potassium permanganate, iodic or periodic acids, or the like, and preferably also includes an expansion aid as described below. Although less preferred, the intercalant may contain metal halides such as ferric chloride, and ferric chloride mixed with sulfuric acid, or a halogen, such as bromine as a solution of bromine and sulfuric acid or bromine in an organic solvent.
In accordance with the present invention the particles of graphite flake treated with intercalant are contacted e.g. by blending, with a reducing organic agent selected from alcohols, sugars, aldehydes and esters which are reactive with the surface film of oxidizing intercalating solution at temperatures in the range of 25xc2x0 C. and 125xc2x0 C. Suitable specific organic agents include the following: hexadecanol, octadecanol, 1-octanol, 2-octanol, decylalcohol, 1, 10 decanediol, decylaldehyde, 1-propanol, 1,3 propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose, lactose, sucrose, potato starch, ethylene glycol monostearate, diethylene glycol dibenzoate, propylene glycol monostearate, propylene glycol monooleate, glycerol monostearate, glycerol monooleate, dimethyl oxylate, diethyl oxylate, methyl formate, ethyl formate and ascorbic acid.
Also effective are polyfunctional compounds, e.g., those having both surfactant qualities and more than one reducing function selected from the group consisting of alcohols, esters, aldehydes and the like. One example is lignin-derived compounds, such as sodium lignosulfate. The preferred compounds are preferably liquid at application temperature and essentially free of water. Among the suitable polyfunctional compounds in this group are surfactants derived from ethylene oxide and/or propylene oxide and a compound capable of contributing a hydrophobic group to the compound, e.g., polymers of ethylene oxide and nonylphenol available as Tergitol NP detergents, products formed by the reaction of linear secondary alcohols with ethylene oxide available as Tergitol 15-S- detergents, and various alkylaryl polyether alcohols prepared by the reaction of octylphenol with ethylene oxide as are available as Triton X detergents. Examples are presented below of materials effective as reducing organic agents that can improve both free and compressed expansion.
The amount of organic reducing agent is suitably from about 0.5 to 4% by weight of the the particles of graphite flake. The use of an expansion aid applied prior to intercalation or during intercalation can also provide improvement. Among these improvements can be reduced exfoliation temperature, and increased expanded volume (also referred to as xe2x80x9cworm volumexe2x80x9d).
An expansion aid in this context will be an organic material sufficiently soluble in the intercalant solution to achieve an improvement in expansion. More narrowly, organic materials of this type that contain carbon, hydrogen and oxygen, preferably exclusively, may be employed. Carboxylic acids have been found effective in this invention. A suitable carboxylic acid as the expansion aid can be selected from aromatic, aliphatic or cycloaliphatic, straight chain or branched chain, saturated and unsaturated monocarboxylic acids, dicarboxylic acids and polycarboxylic acids which have at least 1 carbon atom, and preferably up to about 10 carbon atoms, which is soluble in the aqueous intercalant solution employed according to the invention in amounts effective to provide a measurable improvement of one or more aspects of exfoliation. Preferred products are characterized by an intumescent temperature of below about 200xc2x0 C. According to some observations, exfoliation can be initiated at temperatures as low as 160xc2x0. Suitable water-miscible organic solvents can be employed to improve solubility of an organic expansion aid in the intercalant solution.
Representative examples of saturated aliphatic carboxylic acids are acids such as those of the formula H(CH2)nCOOH wherein n is a number of from 0 to about 5, including formic, acetic, propionic, butyric, pentanoic, hexanoic, and the like. In place of the carboxylic acids, the anhydrides or reactive carboxylic acid derivatives such as alkyl esters can also be employed. Representative of alkyl esters are methyl formate and ethyl formate. Sulfuric acid, nitric acid and other known aqueous intercalants have the ability to decompose formic acid, ultimately to water and carbon dioxide. Because of this, formic acid and other sensitive expansion aids are advantageously contacted with the graphite flake prior to immersion of the flake in aqueous intercalant.
Representative of dicarboxylic acids are aliphatic dicarboxylic acids having 2-12 carbon atoms, in particular oxalic acid, fumaric acid, malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid, 1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid, 1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid and aromatic dicarboxylic acids such as phthalic acid or terephthalic acid. Representative of alkyl esters are dimethyl oxylate and diethyl oxylate. Representative of cycloaliphatic acids is cyclohexane carboxylic acid and of aromatic carboxylic acids are benzoic acid, naphthoic acid, anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- and p-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoic acids and, acetamidobenzoic acids, phenylacetic acid and naphthoic acids. Representative of hydroxy aromatic acids are hydroxybenzoic acid, 3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4-hydroxy-2-naphthoic acid, 5-hydroxy-l-naphthoic acid, 5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and 7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids is citric acid.
The intercalant solution will be aqueous and will preferably contain an amount of expansion aid of from about 1 to 10%, the amount being effective to enhance exfoliation. In the embodiment wherein the expansion aid is contacted with the graphite flake prior to immersing in the aqueous intercalant solution, the expansion aid can be admixed with the graphite by suitable means, such as a V-blender, typically in an amount of from about 0.2% to about 10% by weight of the graphite flake. After intercalating the graphite flake with an intercalating solution, preferably containing an expansion aid, and following the blending of the intercalant coated intercalated graphite flake with the organic reducing agent, the blend is exposed to temperatures in the range of 25xc2x0 to 125xc2x0 C. to promote reaction of the reducing agent and intercalant coating. The heating period is up to about 20 hours, with shorter heating periods, e.g., at least about 10 minutes, for higher temperatures in the above-noted range. Times of one half hour or less, e.g., on the order of 10 to 25 minutes, can be employed at the higher temperatures.